Various electromagnetic techniques exist to perform surveys of a subterranean structure underneath a surface for identifying layers of interest. Examples of layers of interest in the subterranean structure include subsurface resistive bodies, such as hydrocarbon-bearing reservoirs, gas injection zones, thin carbonate or salt layers, and fresh-water aquifers. One survey technique is a magnetotelluric (MT) survey technique that employs time measurements of electric and magnetic fields (which are responsive to naturally occurring electromagnetic fields) for determining the electrical conductivity distribution beneath the surface. Another survey technique is a controlled source electromagnetic (CSEM) survey technique, in which an electromagnetic transmitter, referred to as a “source,” is used to generate electromagnetic signals. With either survey technique, surveying units, referred to as “receivers,” are deployed on a surface (such as at the sea floor or on land) within an area of interest to make measurements from which information about the subterranean structures can be derived. The receivers may include a number of sensors for detecting any combination of electric fields, electric currents, and magnetic fields.
In marine environment CSEM surveys, modeling and acquisition studies have shown that thin resistive targets in a subterranean structure, such as hydrocarbon-bearing reservoirs, gas injection zones, thin carbonate or salt layers, fresh water aquifers, and so forth, are more easily detectable when a CSEM source is positioned close to the sea floor. In practice, the CSEM source is towed or “flown,” as close to the sea floor as the conditions will allow. Typically, the CSEM source will be towed between 30 to 50 meters above the sea floor.
In a deep water marine environment, locating the EM source close to the sea floor can make the EM survey difficult and expensive for various reasons. For example, the EM source has to be hardened to withstand pressures experienced in deep water. In addition, CSEM sources require a large amount of current to create the necessary moment, and the current results in a significant amount of energy that is lost due to resistive heating along a tow cable, which can be up to 6,000 meters in length. Also, in a deep water application, the speed of the sea vessel cannot exceed 1 to 2 knots to prevent the CSEM source from rising towards the sea surface and away from the sea floor.